Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and must be used at high concentrations. As a result, it is sometimes necessary to remove catalyst residues from the polymer, which adds to production costs. Furthermore, Zeigler-Natta catalysts typically produce polymers having high densities and broad molecular weight distributions, properties that are undesirable for some applications such as injection molding. They are also generally poor at controlling polymer density through incorporation of .alpha.-olefin comonomers. Unfortunately, when comonomers are used, they are distributed in a non-uniform fashion among the different molecular weights that comprise the molecular weight distribution. Most of the comonomer is incorporated into the low molecular weight polymer molecules; a more uniform incorporation would be desirable.
To improve polymer properties, highly active single-site catalysts, in particular metallocenes, are beginning to replace Zeigler-Natta catalysts. Although more expensive, the new catalysts give polymers with narrow molecular weight distributions, low densities, and good comonomer incorporation.
A metallocene catalyst consists of one or more cyclopentadienyl ring ligands bound to a transition metal in an .eta..sup.5 fashion. The cyclopentadienyl ring ligands are polymerization-stable; that is, they remain bound to the metal during the course of the polymerization. One disadvantage of metallocene catalysts is that they tend to produce lower molecular weight polymers at higher temperatures.
Recent attention has focused on developing improved single-site catalysts in which a cyclopentadienyl ring ligand of the metallocene is replaced by a heteroatomic ring ligand. These catalysts may be referred to generally as heterometallocenes.
In particular, U.S. Pat. No. 5,554,775 discloses catalysts containing a boraaryl moiety such as boranaphthalene or boraphenanthrene. Further, U.S. Pat. No. 5,539,124 discloses catalysts containing a pyrrolyl ring, i.e., an "azametallocene." In addition, PCT Int. Appl. WO 96/34021 discloses azaborolinyl heterometallocenes wherein at least one aromatic ring includes both a boron atom and a nitrogen atom.
Metallocenes and heterometallocenes are much more expensive to produce than the Zeigler-Natta catalysts. Therefore, further research has focused on developing less expensive single-site catalysts that give advantageous polymer properties. One approach is to use readily available organic compounds that can act as polymerization-stable, anionic ligands for transition metals. For example, U.S. Pat. No. 5,637,660 discloses catalysts in which a cyclopentadienyl moiety of a metallocene is replaced by a readily available quinolinyl or pyridinyl ligand. Other inexpensive organic ligands capable of binding a transition metal may also be available. One example is hydroxylamine derivatives. Hughes, et al., J. Chem. Soc., Dalton Trans. (1989) 2389, for example, describe the crystal structure of organometallic compounds containing hydroxylamine or hydrazine derivatives bound to a titanium complex in an .eta..sup.2 fashion, but they do not describe olefin polymerization catalysts.
In sum, new single-site catalysts are needed. Particularly valuable catalysts would be easily synthesized from readily available starting materials. These catalysts would combine the cost advantages of Zeigler-Natta catalysts with the polymer property advantages of single-site catalysts.